Wellhead blowout preventers are used in oil wellhead assemblies to seal against a central bore. In a pumping production wellhead, the blow out preventer (BOP) may be configured to seal against a polish rod or a tubular member if present. The rams of a BOP may operate in different ways in closing off a well, or multiple BOPs may be used in a wellhead stack to provide different functions. In a production pumping well, opposing polish rod BOP rams accommodate and seal around a polish rod extending generally vertically through the wellhead. Blind BOP rams seal against each other across the central bore when no polish rod is in place.
A typical prior art BOP for a production wellhead is disclosed in U.S. Pat. No. 5,765,813 to Lam et al., issued Jun. 16, 1998, owned by Stream-Flo Industries Ltd., the assignee of this patent application. This type of BOP is commonly used in connection with pumping production wells. With such wells, a sucker rod string is reciprocated or rotated to drive a downhole pump, which lifts the produced fluid to the surface through a tubing string. The BOP is equipped with a pair of opposing polish rod rams which can be advanced horizontally to seal around the vertical polish rod portion of the rod string to prevent the upward escape of fluid. Alternatively, if the rod string is out of the well, the inner or front ends of the rams can be pressed together to cause closure of the wellhead assembly fluid passageway.
More particularly, a typical pumping production BOP includes a cross-shaped housing forming a central, generally vertical bore and a pair of coaxial, horizontal ram bores intersecting the central bore from each side. The BOP is commonly positioned in the wellhead assembly between the tubing head and flow tee. In this configuration, the central bore of the BOP forms part of the wellhead assembly fluid passageway. Within the BOP, a pair of rams is positioned in the horizontal ram bores. Actuator mechanisms, which generally include rams screw powered by mechanical, electrical, pneumatic or hydraulic actuators, are provided at the outer ends of the ram bores, for extending or retracting the rams into or out of the central bore, in order to close or open the central well bore. Each BOP ram comprises a generally cylindrical body, although other shaped ram bodies are known (ex. oval, rectangular or square in cross section). The ram generally comprises a steel core, preferably having an outer full bore diameter portion (or rear portion) and a reduced diameter inner portion (or front portion). The ram core inner portion is covered with and bonded to a layer of an elastomeric material, typically a nitrile rubber. The ram bores, typically cylindrical, extend into the central bore and the bore surfaces combine at their intersection to form sealing areas. When the rams move into the central bore, the rubber surfaces of their inner portions seal against the sealing surfaces.
The rubber-coated inner or front face of each ram is typically formed to provide a semi-circular, vertically directed groove, also termed vertical radial groove. When the polish rod of the rod string is present in the central bore, opposing ram ends encircle and press against the polish rod to form a seal of the central bore. When the polish rod is not present the ram ends compress together to form a solid block. In both cases, the circumferential seals of the ram side surfaces, with the sealing areas and the end face seals, combine to close the central bore and contain pressurized fluids.
U.S. Pat. No. 7,673,674, issued Mar. 9, 2010 to Tony M. Lam, and assigned to Stream-Flo Industries Ltd., the assignee for this patent application, describes a BOP ram and/or polish rod claim in which the vertically directed groove at the front face of the ram is shaped as a V-groove to accommodate a portion of the circumference of the polish rod. U.S. Pat. No. 7,552,765 to Tony M. Lam, issued Jun. 30, 2009, and assigned to Stream-Flo Industries Ltd., describes a BOP in which one of the rams is formed with an extended central bore sealing section behind the front sealing end of the ram, such that the extended central bore sealing section can be extended across the central bore to seal the central bore when the polish rod is not present.
In a production BOP, pressure acting from below on the closed BOP rams may extrude the side rubber upwardly so that the circumferential seal with the sealing areas is lost. As well, the end rubber bonded to the vertical end faces (front faces) of the ram cores may tear loose from the core when high pressure is exerted from below. In severe conditions, such as injecting chemicals to close off a well, the rubber degrades quickly, causing the seals to fail. A large number of BOP ram designs exist to address problems of extrusion, tearing or degradation of the BOP ram seals.
Canadian Patent Application No. 2,260,655, published Aug. 2, 2000 naming Tony M. Lam and Keith D. Farquharson as inventors and Stream-Flo Industries Ltd. as assignee, describes a ram type BOP for high temperature applications. The BOP incorporates a generally L-shaped seal element formed of a graphite or asbestos seal material, which is sandwiched between a bottom L-shaped steel retainer plate and a semi-cylindrically shaped steel top retainer plate. This design of BOP ram is well suited for brittle seal materials such as graphite and asbestos when higher temperatures are encountered.
Other ram type BOP devices exist which use graphite or asbestos type seals or seal inserts. However, graphite and asbestos type sealing materials typically include polymeric bonding materials which have lower temperature limits than graphite or asbestos, so the seals are prone to failure at very high temperatures, such as temperatures above 600° F. Today, high temperature wellhead applications such as steam injection may require the wellhead equipment to accommodate temperatures above 600° F. Also, the use of asbestos seal materials is being phased out in industry due to harmful effects of asbestos fibres in handling, manufacture and in use.
Thermoplastic materials having superior chemical resistance are available, but have different compressibility and elastic properties than elastomeric sealing materials such as nitrile rubbers. Thus, BOP devices are not generally amenable to simple substitution of thermoplastics for the rubber sealing components. U.S. Pat. No. 7,137,610, issued to Lam on Nov. 22, 2006, and assigned to Stream-Flo Industries Inc., describes a production BOP and BOP rams incorporating thermoplastic seals at the front of the rams to seal against the polish rod. However, even thermoplastic materials have temperature limits below the temperatures encountered for steam injection wellheads.
In spite of the above advances in BOP ram seals, there is still a need for BOP devices capable of withstanding very high or very low temperature environments. For instance, in wellheads through which steam is injected to enhance recovery in depleted wells or for heavy oil wells, temperatures in excess of 650° F. can be reached. These temperatures far exceed the limits of nitrile rubber seals, elastomeric seal materials, and thermoplastic seal materials. As above, BOP devices adapted to carry graphite or asbestos seals are also subject to failure at such extreme temperatures as polymeric bonding materials in the seals begin to break down. As well, at very low temperatures, conventional BOP sealing materials become very hard or brittle, interfering with the ability to make reliable seals.
There is a still a need for a ram type BOP which can reliably seal against a polish rod and/or the central bore of a wellhead in very high temperature and very low temperature applications.